DE102012014225A1 - Adhesive useful for highly porous ceramics for producing diesel particulate, comprises phosphate binder, mullite, cellulose, clay, fine-grained silicon carbide powder and coarse-grained silicon carbide powder - Google Patents

Abstract

Adhesive for highly porous ceramics for producing diesel particulate from silicon carbide segments for coating and gluing a honeycomb body made of at least two segments with each other and for sealing the channels in opposite directions on at least one side, comprises phosphate binder, mullite, cellulose, preferably hydroxyethylcellulose, clay, fine-grained silicon carbide powder and coarse-grained silicon carbide powder. The adhesive is fiber-free. The phosphate binder is used as a temperature resistant binder. Mullite is used as reactants for phosphate binders. Adhesive for highly porous ceramics for producing diesel particulate from silicon carbide segments for coating and gluing a honeycomb body made of at least two segments with each other and for sealing the channels in opposite directions on at least one side, comprises phosphate binder, preferably monoaluminum phosphate, 0.1-20%, preferably 8% mullite, 0.01-5% cellulose, preferably hydroxyethylcellulose, clay, 5-60%, preferably 35% fine-grained silicon carbide powder and 5-50%, preferably 30% coarse-grained silicon carbide powder. The adhesive is fiber-free. A phosphate binder, preferably monoaluminum phosphate is used as a temperature resistant binder. Mullite is used as reactants for phosphate binders. Cellulose, preferably hydroxyethyl is used for water retention. The thermal conductivity of the adhesive is 40-80 W/mx K in the used temperature range. The specific heat capacity of the adhesive is 0.6 kJ/ kgx K at 20[deg] C, and increases to 1 kJ/kgx K at 1000[deg] C. The heat capacity of the adhesive per volume unit is greater than 40-60% than the heat capacity of the ceramic and so represents the heat sink during regeneration. The thermal expansion coefficient of the adhesive in the application area lies close to the honeycomb ceramic and does not deviate from more than 5%. The coarse-grained silicon carbide powder has a maximum grain size of 1 mm, preferably 500 mu m. The fine-grained silicon carbide powder exhibits a maximum particle size of 100 mu m. The mullite has a maximum grain size of 200 mu m, preferably 100 mu m. The phosphate binder is monoaluminum phosphate (preferred), phosphoric acid, monomagnesium phosphate, alkali phosphate, boron monophosphate or sodium polyphosphate. The clay is derived bentonite, kaolinite and/or fire clay.

Description

field of use

For the filtration of liquid or gaseous substances more and more ceramic parts are used. Since these parts can not be made crack-free in such large dimensions, because shrinkage occurs during sintering, the parts are made of individual segments and glued together. The ceramics may come from the field of oxidic substances or come from the field of non-oxidic substances, such as SiC or Si ₃ N ₄ or mixtures thereof. The segments have a certain porosity, so that the finest molecules can go through the wall such. As air or exhaust gas. This filtration can be diesel particulate filters, which are assembled from individual honeycomb segments and glued together. The invention is an adhesive which can also be used to seal individual channels and even provide advantages for wrapping the parts.

State of the art

So far, many patents coming from Japan are described so that the adhesive for DPF consists of SiC powder as a filler, a silica sol as an inorganic binder, inorganic fibers and an organic binder. The adhesive is to connect the segments together and thermally isolate. The following applications are described in detail:
In EP 0816065 A1 In the field of diesel particulate filters, a material for bonding and clogging of the channels is described, which should be elastic. This is very difficult for ceramic materials. The binder used is a colloidal liquid (colloidal sol). However, this only provides insufficient inherent strength and low adhesion to the ceramic. Because the binder consists of nanoparticles, a strength is achieved only after sintering, which is reasonably necessary for such adhesives. Nevertheless, it is at the lower limit. In order to achieve a minimum strength, fibers or whiskers must still be added to the material in order to achieve at least the necessary tensile strength. This is not harmless to the environment during processing and in subsequent use fibers could also be released from the mass. SiC is used as the filler, which seems to be suitable and necessary when using diesel particulate filters made of SiC components. The use of cellulose as an organic adjuvant is processing technology help, because a so-called green strength is achieved. The fiber length is given as 1 to 100 mm, which is not manageable in reality. How should a processing of a mass with such long fibers be accomplished.

In EP 1 142 619 B1 The same composition as described above is described with SiC powder, SiO ₂ -based colloidal sol, organic binder and inorganic fibers. The inorganic fibers are derived from the system SiO ₂ -Al ₂ O ₃ . These fibers have much lower thermal conductivity than z. As SiC particles and act as an insulator. For a sealing and sealing layer that may not be wrong, but for a glue this is the wrong choice. This is reported with values of WLF of 0.1 to 10 W / m.times.K for the adhesive and sealant layer as compared to 20 to 80 W / m.times.K for the SiC ceramic itself, which was named.

In EP 1 270 202 B1 the filler of the adhesive is considered and the α-type SiC is highlighted. This is a necessity, otherwise conversions would occur at application temperatures up to 1200 ° C.

In EP 1 306 358 B1 Again, the same adhesive is described with SiC as filler, an inorganic binder based on a colloidal sol based on SiO ₂ , an organic binder based on polysaccharide and / or cellulose and an inorganic fiber. With this fiber it is emphasized that the shot content should amount to only 1 to 10%. The fibers come from the range SiO ₂ and Al ₂ O ₃ . The indication of the shot content is clearly proof that it is glassy / amorphous fibers that will undergo a transformation under temperature and lose their elasticity. They crystallize. The silica sol brings only after sintering a ceramic bond and thus a necessary strength.

In EP 1 382 445 A2 again the same mass is described for bonding. Again whiskers with a size of 0.1 to 10 or 15 microns and a proportion of 20 to 40% are addressed, alternatively inorganic powder. Whiskers are suspected of causing cancer. For SiC-based fillers it is alleged that if they are larger than 100 μm, the bond strength would be reduced. With gaps of 1 to 3 mm between the individual SiC elements, coarser powder must be present to ensure structural strength.

• • Geringe spezifische Wärmekapazität mit Werten von nur 20 bis 90% der Wärmekapazität der Keramik
• • Eine gute Wärmeisolation, hervorgerufen durch Zugabe von Porenbildner zur Masse, um im gesinterten Zustand viele Poren zu erzeugen. Dieser Gedankengang ist falsch, weil bei dem Abbrand des Rußes punktuell hohe Temperaturen auftreten können. Eine hohe Wärmeleitfähigkeit verteilt die Energie schnell und lässt erst gar nicht hohe Temperaturspitzen zu. Wenn zudem noch eine hohe Wärmekapazität gegeben ist, unterstützt dies den Weg der „Wärmesenke zw. Wärmeabfuhr”.
• • Zudem soll die Wärmedehnung des Klebers unterschiedlich sein zu der Wärmedehnung der Keramik. Wären sie identisch oder fast identisch, nähme die Stabilität der Verklebungsschicht durch Thermoschockreaktionen stark ab. Der Einwand dagegen lautet, dass bei Verbindungen mit unterschiedlicher Wärmedehnung an der Übergangsstelle Spannungen auftreten, die zur Rissbildung führen können. Dies wird noch verstärkt, wenn beim Abbinden des Klebers Schwindungen auftreten sollten. Das Gegenteil ist der Fall: Die Wärmedehnung sollte so ähnlich wie möglich sein, um mechanische Spannungen zu minimieren.

In EP 1 479 881 A1 Again, the adhesive of SiC powder, colloidal sol, inorganic fibers and organic binder is discussed. The adhesive should have the following properties:

• • Low specific heat capacity with values of only 20 to 90% of the heat capacity of the ceramic
• • A good thermal insulation, caused by the addition of pore-forming agent to the mass to produce many pores in the sintered state. This train of thought is wrong, because at the burnup of the soot punctually high temperatures can occur. A high thermal conductivity distributes the energy quickly and does not even allow high temperature peaks. If, in addition, there is still a high heat capacity, this supports the path of the "heat sink between heat removal".
• • In addition, the thermal expansion of the adhesive should be different to the thermal expansion of the ceramic. If they were identical or almost identical, the stability of the bond layer would be severely reduced by thermal shock reactions. The objection, on the other hand, is that, in the case of compounds with different thermal expansion at the transition point, stresses occur which can lead to the formation of cracks. This is exacerbated when shrinkage should occur when the adhesive sets. The opposite is the case: the thermal expansion should be as similar as possible to minimize mechanical stress.

In EP 1 719 881 A2 is again addressed to a necessary porosity of the adhesive in order to keep the heat capacity low and the thermal conductivity low. This approach is wrong in our eyes. In addition, there is the claim that the thermal expansion of the adhesive could be influenced by the porosity of the adhesive. With pores, no thermal expansion can be changed, otherwise the porous or highly porous SiC ceramics for the diesel soot filters would have to have a completely different thermal expansion than dense SiC ceramics.

Disadvantages of the prior art

In all the mentioned patent applications reference is made to inorganic fibers or whiskers. Since the asbestos fibers have already made very bad experiences, one would like to get away from such materials in the future. They pollute the environment during processing and endanger the employees who have to handle the fibers. But they are a necessity when using silica sols as an inorganic binder to obtain sufficient strength. Since gas flows occur in the filters, some tensile strength must be present and this is achieved by the fibers. The colloidal silica sol consists of a liquid and finest amorphous particles. These particles have a very large specific surface area and can sinter early. But the disadvantage is that shrinkage already occurs due to the large amount of water during drying. The next point is that the strength is first generated by sintering these finest SiO ₂ particles. This goes along with the sintering shrinkage. The prefabricated segments are held together by a shrinking adhesive, which already leads to tensions in thermal equilibrium.

Object of the invention

• • Die Masse soll keine Fasern enthalten
• • Sie soll temperaturbeständig bis 1200°C sein
• • Sie soll ähnliche Wärmedehnung wie die SiC-Keramik haben
• • Sie soll eine hohe Wärmeleitfähigkeit haben
• • Sie soll eine große spezifische Wärmekapazität aufweisen
• • Sie soll eine feste Anhaftung an die SiC-Keramik gewährleisten
• • Sie soll eine gute Temperaturwechselbeständigkeit haben

A material composition for an adhesive is to be found which has the following properties:

• • The mass should not contain fibers
• • It should be temperature resistant up to 1200 ° C
• • It should have similar thermal expansion as the SiC ceramic
• • It should have a high thermal conductivity
• • It should have a high specific heat capacity
• • It should ensure a firm adhesion to the SiC ceramic
• • It should have a good thermal shock resistance

General solution

• • Kolloidale Sole wie bereits besprochen, haben aber zu geringe Festigkeit
• • Hydraulische Bindemittel z. B. auf Zementbasis verlieren zu früh die Festigkeit, bevor die keramische Bindung einsetzt. Zudem müsste zuviel artfremdes Material auf Basis Bindeton verwendet werden, was eine Verschlechterung bzgl. Wärmedehnung ergäbe und die Gefahr von Kristallumwandlungen
• • Wasserglas bringt aber die Gefahr, dass die vorhandenen Alkalien eine frühzeitige Bildung einer Schmelze begünstigen könnten
• • Phosphatbinder sind geeignet

For temperature-resistant adhesives up to 1200 ° C, the following products are available as stable binders:

• • Colloidal sols as discussed earlier, but have too low strength
• • Hydraulic binders z. As cement based lose too early the strength before the ceramic bond begins. In addition, too much foreign material would have to be used based on binding clay, which would result in a deterioration in terms of thermal expansion and the risk of crystal transformations
• • Water glass, however, brings with it the danger that the existing alkalis could favor the early formation of a melt
• • Phosphate binders are suitable

With a phosphate binder there is a suitable binder for this temperature application. If a suitable reactant is added, excellent strength can be achieved even at very low temperature. This reaction partner can come from the range of oxide ceramics such as MgO, Al ₂ O ₃ , mullite, spinel or other oxides. As the filler, SiC, Si ₃ N ₄ or a mixture thereof is used because of thermal expansion and thermal conductivity. In addition, water is added to adjust the viscosity, and in order to have sufficient water retention, a cellulose is added.

SiC is added in different grain sizes. Coarser SiC forms a scaffold and causes a high thermal conductivity. Fine-grained SiC fills in the interstices, reduces the porosity of the adhesive and thus increases the specific heat capacity per volume. This again increases the thermal conductivity. The phosphate binder reacts in the lower temperature range due to its acidic properties with oxidic raw materials (reactants) by a neutralization reaction.

Detailed description

• • Die Wärmeleitfähigkeit liegt bei ca. 40 bis 80 W/m × K im verwendeten Temperaturbereich
• • Die spezifische Wärmekapazität liegt bei ca. 0,6 kJ/kg × K bei 20°C und steigt bis auf über 1,0 kJ/kg × K. Pro Volumeneinheit ist die Wärmekapazität des Klebers um 40 bis 60% größer als die Wärmekapazität der Wabenkeramik
• • Der thermische Ausdehnungskoeffizient liegt im Anwendungsbereich sehr nah an der Wabenkeramik und weicht nicht mehr als 5% davon ab.

The adhesive described in the invention has the following properties:

• • The thermal conductivity is about 40 to 80 W / m × K in the temperature range used
• • The specific heat capacity is approx. 0.6 kJ / kg × K at 20 ° C and rises above 1.0 kJ / kg × K. Per volume unit, the heat capacity of the adhesive is 40 to 60% greater than the heat capacity the honeycomb ceramic
• • The coefficient of thermal expansion in the field of application is very close to the honeycomb ceramic and does not deviate more than 5% from it.

These properties of the adhesive are determined by a mixture of inorganic powders, preferably SiC with a phosphate binder and a reactant, for. B. Mullite achieved. It is followed by a heat treatment, wherein the phosphate binder cures with the mullite by a neutralization reaction.

The individual components are shown:

SiC coarse grained:

This SiC builds up the main structure. The large grains make it possible to set the highest possible thermal conductivity. Instead of SiC, Si ₃ N ₄ can also be used. Grain size range in general: 0-500 μm Flow range: 10-50%

SiC fine grained:

The fine grain is intended to fill up the spaces between the large grains and thus to keep the porosity of the connecting layer as low as possible. Instead of SiC, Si ₃ N ₄ can also be used. Grain size range in general: 0-100 μm Flow range: 10-40%

mullite:

The mullite is the reaction partner for the phosphate binder. Flow range: 0.1-20%

Volume:

Additional reaction partner for the phosphate binder. The term clay can be used to refer to bentonite, kaolinite, fireclay, montmorillonite and illite. Flow range: 0.01 to 5%

Phosphate binders:

It is a monoaluminum phosphate used whose reaction with the mullite begins only at higher temperatures. Flow range: 1 to 40%

Water:

With the addition of water, the viscosity of the adhesive is adjusted. Flow range: 1 to 40%

cellulose:

The cellulose retains water in the adhesive and prevents it from being removed from the adhesive too quickly when the adhesive is applied to a highly porous honeycomb segment. Flow range: 0.01 to 10%

embodiment

For the bonding of porous SiC filter elements, a mixture of the following substances was put together: component properties Quantity in g SiC large grain <350 μ 28 SiC fine-grained (1) F 400 21 SiC fine grained (2) F 500 42 mullite <100 μm 8th volume <50 μm 2 Mono aluminum phosphate 50% strength 20 cellulose HEC 1 g H ₂ O 20 g

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0816065 A1 [0002]
• EP 1142619 B1 [0003]
• EP 1270202 B1 [0004]
• EP 1306358 B1 [0005]
• EP 1382445 A2 [0006]
• EP 1479881 A1 [0007]
• EP 1719881 A2 [0008]

Claims (1)

Adhesive for highly porous ceramics for the production of diesel particulate filters from SiC segments, in order to glue a honeycomb body of at least 2 segments together, to encase and to close the channels in opposite directions on at least one side, characterized by the following features: 1) The glue is fiber-free 2) Used as a temperature-resistant binder is a phosphate binder, in the specific case a monoaluminum phosphate 3) A suitable mullite is used as the reaction partner for the phosphate binder 4) For the water retention, a suitable cellulose is used, here especially a hydroxyethyl cellulose 5) The thermal conductivity of the adhesive is 40 to 80 W / m × K in the temperature range used. 6) The specific heat capacity is about 0.6 kJ / kg × K at 20 ° C and up to about 1.0 kJ / kg × K increases at temperatures of 1000 ° C. 7) Per volume unit, the heat capacity of the adhesive is 40 to 60% greater than the heat capacity of the ceramic and thus represents a heat sink during regeneration. 8) The thermal expansion coefficient of the adhesive in the application range is very close to the honeycomb ceramic and does not deviate more than 5% thereof. 9) The coarse-grain SiC powder has a maximum grain size of 1 mm, in this case especially 500 μm 10) The amount of coarse-grained SiC powder can be from 5 to 50%, here 30% 11) The fine-grained SiC powder has a maximum grain size of 100 microns 12) The amount of fine-grained SiC powder can be from 5 to 60%, here 35% 13) The amount of mullite can be from 0.1 to 20%, here 8% 14) The mullite has a maximum particle size of 200 μm, here 100 μm 15) The Phosphatbinder a monoaluminum phosphate, a phosphoric acid, monomagnesium phosphate, alkali phosphates, Bormonophosphat, sodium polyphosphate, here monoaluminum phosphate 16) The cellulose consists of a HEC and is used in amounts of 0.01 to 5% 17) A clay is used that comes from the range bentonite, kaolinite, fireclay or a mixture thereof